Did the Moon or planets form in a manner similar to that of Earth? (Beginner)

Did the moon or planets accrete in ways similar to Earth, in the same time frame, or both? How do the accretion histories of the moon and other planets compare with that of Earth?

It is unlikely that the moon formed in the same way as earth, i.e., as a rocky core in orbit around the juvenile sun, accreting material from debris in the young solar system. A more likely scenario is the moon as a by-product of an impact between early earth (after the core had formed, but before earth had fully accreted) and a Mars-sized rocky body.

Planets between the sun and the asteroid belt are all composed largely of silicate rocks. That is, most of their mass consists of elements and compounds that vaporize at "very high" temperatures. All such terrestrial planets formed in similar ways, leading to similar vertical layers within them (typically core, mantle, and crust). But the material that accreted to make them varied somewhat, possibly in narrow bands, with distance from the young sun. A good example is the difference in the K/Th mass fraction ratio as well as in the mass fraction of Fe between Earth and Mars.

Clear and detailed descriptions of the origin of the moon are available at the following sites:

Planetary Science Discoveries: Earth Moon system origin

Big bang, new Moon

Issues with the impact-origin theory is highlighted in a well-cited Wikipedia entry as well.

An additional concise but insightful description on the origin of the moon is available at:

Planetary Science Institute Moon origin

This page was last updated on February 10, 2016

About the Author

Suniti Karunatillake

After learning the ropes in physics at Wabash College, IN, Suniti Karunatillake enrolled in the Department of Physics as a doctoral candidate in Aug, 2001. However, the call of the planets, instilled in childhood by Carl Sagan's documentaries and Arthur C. Clarke's novels, was too strong to keep him anchored there. Suniti was apprenticed with Steve Squyres to become a planetary explorer. He mostly plays with data from the Mars Odyssey Gamma Ray Spectrometer and the Mars Exploration Rovers for his thesis project on Martian surface geochemistry, but often relies on the synergy of numerous remote sensing and surface missions to realize the story of Mars. He now works at Stonybrook.

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